The future of Diego Garcia, the British-controlled territory in the Indian Ocean used by the CIA for secret rendition flights, will be discussed during an unprecedented meeting next week between David Cameron and Navinchandra Ramgoolam, the prime minister of Mauritius, the Guardian has learned.

The meeting comes at a time when Britain, faced with a string of lawsuits, is under increasingly heavy pressure to return Diego Garcia and the other 54 islands in the Chagos archipelago to Mauritius. Ramgoolam will be in London for the Queen's diamond jubilee celebrations.

Diego Garcia was used by the US to bomb Iraq and Afghanistan and for CIA rendition flights, including one putting a Libyan dissident into the hands of Muammar Gaddafi's secret police in an operation involving MI6.

The island is almost certainly used by the US for long-range bombers and would act as a military communications hub in any attack on Iran. That prospect is causing deep anxiety among British military chiefs, fearful of the consequences for UK forces in Afghanistan and the Gulf.

In an exclusive interview with the Guardian, Milan Meetarbhan, the Mauritian ambassador to the UN and one of his government's top officials, spoke of a "reassertion of [Mauritian] sovereignty over the Chagos islands". That, he said, "can be done now".

The agreement setting up the American base, signed by the US and UK in 1966, expires in 2016. Though it includes a 20-year optional extension, both parties must agree to extend, modify, or end it by December 2014.

The 1,500 Chagossians, of whom 500 lived in Diego Garcia, were expelled between 1968 and 1973 in violation of the UN declaration of human rights.

"Worse," says David Snoxell, former UK high commissioner to Mauritius, "their removal was done in secret to maintain a fiction and deceive the UN that there were no indigenous people on Chagos, only contract workers who belonged to Mauritius and Seychelles."

Snoxell, now co-ordinator of the UK parliament's all-party Chagos Islands group, describes it as "one of the worst violations of fundamental human rights perpetrated by Britain in the 20th century".

The Foreign Office described the aim as building "defence facilities … without hindrance or political agitation", according to recently released Colonial Office archives.

The Labour government of the time offered Diego Garcia to Washington as a military base just as it was negotiating the cost of Polaris nuclear missiles, Trident's predecessor, which it was in the process of buying from the US.

Washington never paid a lease for the base but instead reduced the Polaris bill - a move kept secret from the US Congress as well as the House of Commons.

Chagos islanders have taken their case for the "right to return" to the European court of human rights, which is expected to give its ruling this summer.

"During 12 years of litigation, costing the taxpayer in the region of £4m, some of the original inhabitants have died without being able to see their homeland, their churches and ancestors' graves," said Snoxell.

In 2010, in a move described by Geoffrey Robertson QC, an international constitutional expert and former UN appeal judge, as an "act of environmental imperialism" and by Ramgoolam as "a policy of deceit", the UK established a 200-mile "marine protected area" around the Chagos.

The move was exposed by WikiLeaks and the Guardian as the latest ruse to prevent the Chagos islanders from ever returning to their homeland.

According to a leaked US diplomatic cable, Foreign Office official Colin Roberts "asserted that establishing a marine park would, in effect, put paid to resettlement claims of the archipelago's former residents".

He was quoted as saying: "According to the HMG's [Her Majesty's government's] current thinking on the reserve, there would be 'no human footprints' or 'Man Fridays' on the British Indian Ocean Territory uninhabited islands."

"I feel strongly about a policy of deceit," Ramgoolam told the Guardian, adding that he had already suspected Britain had a "hidden agenda". Mauritius has lodged a document with an international tribunal accusing Britain of breaching the UN convention on the law of the sea. It says Mauritius has the sole right to declare an "exclusive zone" around the British colony.

Mauritius could have an even stronger case, and Britain and the US are even more vulnerable, according to international lawyers, because of the use of Diego Garcia to refuel CIA rendition flights.

"The US always knew that its license to occupy Chagos would be up for renewal in 2016, and it chose to run the risk that its British landlord would remain supine and complaisant if the truth were to emerge about how it had used the base for rendition," Robertson says in a legal opinion to be published in an academic journal next week which Ramgooolam will use in his attempt to persuade Cameron.

International legal obligations, he says, "are of sufficient force to cover rendition, which amounts to a conspiracy of torture, or at least an act of aiding and abetting it."The UK, now it has been made aware of the illegal use of Diego Garcia, has a duty not to renew the lease. It could and, in law should, return possession and control to Mauritius, and allow that nation to take responsibility for guarding against further misconduct rather more effectively than the UK has done in the past."

Next week's meeting between the two prime ministers could mark the breakthrough Mauritius has been awaiting for many years. Its hand may be strengthened now that it has offered to host an international court to prosecute pirates operating in the Indian Ocean and off the Horn of Africa, a move welcomed by Britain.

The country makes clear it is not out to provoke the US. Meetarbhan told the Guardian: "We do not want to challenge the continued use of the military facilities by the US.

"What we want is full sovereignty over the Chagos Islands. It is in the interest of the UK and US that all matters are resolved amicably."

Nick Clegg said before the general election that Britain had a "moral responsibility to allow these people [Chagos Islanders] to at last return home". William Hague, now foreign secretary, said that if elected he would "work to ensure a fair settlement of this long-standing dispute".